1. Field of the Invention
The present invention relates to a device for restricting an amount of rotation and a webbing retractor, and in particular, to a webbing retractor in which the amount of taking-up of a webbing can be made constant by restricting the amount of rotation of a spool which takes up the webbing and a device for restricting an amount of rotation used in this webbing retractor.
2. Description of the Related Art
In a conventional webbing retractor 300 shown in FIG. 7, when an acceleration sensor or the like detects that deceleration of a predetermined value or more has occurred in a vehicle, a pretension portion 304 provided at one end 302A of a torsion bar 302 rotates the one end 302A of the torsion bar 302 in the taking-up direction of the webbing. A spool 306 also rotates integrally with the torsion bar 302 in the taking-up direction of the webbing, and the webbing 308 is taken up onto the spool 306. At this time, rotation of a locking shaft portion 310, which is provided at the inner side of the spool 306, in the withdrawing direction of the webbing is prevented by an unillustrated lock portion.
Thereafter, when a withdrawing force is applied to the webbing 308, the spool 306 rotates in the withdrawing direction of the webbing. Because rotation of the torsion bar 302 is prevented by the locking shaft portion 310 at the other end 302B, the torsion bar 302 is twisted by rotating force applied to the one end 302A and acts as a so-called force limiter.
In this way, a relative difference in rotation is generated between the fixed locking shaft portion 310 and the rotating spool 306. Due to this difference in rotation, a ring 314, which is screwed to a male screw 312 of the locking shaft portion 310, rotates integrally with the spool 306 and moves toward a disk 316 of the locking shaft portion 310. When the ring 314 abuts the disk 316, the movement and rotation of the ring 314 is prevented. Accordingly, the rotation of the spool 306 is limited and the amount of withdrawal of the webbing 308 is constant.
Regarding this webbing retractor 300, in a state in which the ring 314 abuts the disk 316, when a large rotating force is further applied to the spool 306 in the direction in which the webbing is withdrawn and the ring 314 moves toward the disk 316, the side flank of a female screw 318, to which pressure is applied, receives outward force in the radial direction along the side flank of the male screw 312, to which pressure is applied. In this way, there is the concern that the ring 314 will be pulled outwardly in the radial direction, that the female screw 318 will be removed from the male screw 312, and that the spool 306 will rotate freely around the locking shaft portion 310. For this reason, it is necessary to increase the strength of the ring 314 by thickening the ring 314, forming the ring 314 from a material having high rigidity, or the like. Accordingly, the ring 314 is heavier and the manufacturing thereof takes much time and labor.
This problem is not only applied to the above-described webbing retractor 300 but is also applied, in general, to a device for restricting an amount of rotation in which a moving body screwed to a shaft body moves rotatingly in the axial direction and the amount of rotation is restricted to a predetermined amount by restricting this axial direction movement.